1. Field of the Invention
This invention relates to cooling equipment for electronic systems, e.g., fans, and more particularly, to controlling the rotational speed of a fan.
2. Description of the Related Art
Fans are often used to evacuate warm air from enclosures in which electronic systems are contained. For example, most computer systems include one or more cooling fans to aid in circulating the air inside the enclosures and for maintaining the temperature inside the enclosures within an acceptable range. The increased airflow provided by fans typically aids in eliminating waste heat that may otherwise build up and adversely affect system operation. Employing cooling fans is especially helpful in ensuring proper operation for certain central processing units (CPUs) with relatively high operating temperatures.
Control of fans in a system typically involves a fan control unit executing a fan control algorithm. A fan control algorithm may determine the method for controlling one or more fans that are configured to evacuate warm air from a system enclosure. For example, the fan control algorithm may specify that a fan's speed should be increased or decreased dependent upon a detected temperature. Such control algorithms may also involve turning off a fan if the temperature is deemed cool enough to do so.
For detecting the temperature, a temperature sensor may provide to the fan control unit a signal indicative of the current temperature of a particular temperature zone in the electronic system. Often, fans used for CPU and/or computer system cooling have a three-wire interface with wires for power, ground, and a tachometer signal. Fan drive systems often use a signal generator that provides a Pulse Width Modulated (PWM) signal to drive an external circuit that controls the voltage between the power and ground interfaces of the fan, which in turn controls the speed of the fan. Signal generators that provide PWM signals are useful because they provide a digital control for the pulse width of a signal. The fan is typically powered only for the duration of the pulse. Between pulses power to the fan is turned off, although the fan is typically still spinning during this time. The duty cycle of the PWM pulse train currently being provided to the fan determines the fan's speed.
One problem that results from using PWM signals to drive fan circuits is that multiple cooling zones create the need for multiple sensors and multiple fans thereby causing the need for complex management schemes in fan operation management. An autofan control algorithm was defined in an industry specification (Heceta 6 Specification, Version .97, May 17, 2001, Intel Corporation) to control three PWM outputs based on three temperature zones. Pursuant to the specification, each temperature zone corresponds to a temperature sensor in a fixed manner. For example, Zone 1 may correspond to temperature measured across the pn-junction of a first remote diode, Zone 2 may correspond to ambient temperature, and Zone 3 may correspond to temperature measured across the pn-junction of a second remote diode. Each PWM signal can be controlled by one zone, or the “hottest” of two or three zones. In other words, control of the device may be performed using sensor information from a selected single corresponding zone, or using sensor information from the hottest of the three zones. If the device is configured for one of the hottest options, the duty cycle for the PWM output will be calculated for each zone with its associated limits and parameters. The zone that produces the highest duty cycle will typically control the PWM output.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.